The United Nations Security Council voted Tuesday to temporarily raise the number of UN peacekeeping troops in South Sudan from 7,000 to 12,500.
The resolution was passed unanimously.
The council also authorized transferring troops and equipment from the Congo, Darfur, Abyei, Liberia and the Ivory Coast to South Sudan.
Condemning the violence against civilians, the council called for "an immediate cessation of hostilities and the immediate opening of a dialogue."
South Sudan army retakes Bor
South Sudan's army stormed the rebel-held town of Bor earler on Tuesday, sending insurgents fleeing nearly a week after they captured the state capital of Jonglei, Information Minister Michael Makwei told AFP.
"The army captured Bor around sunset and the rebel forces are now on the run... We are back in control," Makwei said.
"This is the gift of the government of South Sudan to the people."
However, while claiming the army was now back in full control, the Makwei also said that "shooting continued," without giving further details.
Bor's capture, apparently without major resistance by the rebels, lifts nearly a week-long seige of the town, where some 17,000 civilians fled into the overstretched United Nations peacekeeping compound for protection, severely stretching limited food and supplies.
UN peacekeepers had spent days bolstering fortifications ahead of the army assault.
The UN's humanitarian chief in the country, Toby Lanzer, told reporters, "Absolutely no doubt in my mind that we're into the thousands" of dead.
On Tuesday, the UN said around 34 bodies had been discovered in a mass grave in rebel-held Bentiu, the capital of South Sudan's oil-rich Unity State, following a surge of violence in the world's youngest nation.
"We have discovered a mass grave in Bentiu, in Unity State, and there are reportedly at least two other mass graves in Juba," UN rights chief Navi Pillay said in a statement.
Pillay expressed "grave concern" over the ethnically-tinged killings that have raged across South Sudan for more than a week as troops loyal to President Salva Kiir battle those backing his rival Riek Machar, a former vice president who was sacked in July.
The official toll is 500 dead, although the real figure is believed to be far higher, aid workers say. Hundreds of thousands of others are believed have fled to the countryside, prompting warnings of an imminent humanitarian disaster.
"Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," Pillay said.
"There need to be clear statements and clear steps from all those in positions of political and military control that human rights violations will not be tolerated and those responsible will be brought to justice."
UN chief Ban Ki-moon has asked the Security Council to nearly double the size of the UN mission in the country.
Meanwhile, the UN said at least 45,000 South Sudanese civilians have sought protection at badly overstretched UN bases amid brutal fighting that has spread to half of the young nation's 10 states.
At least 20,000 people are sheltering at two UN bases in the capital Juba, while 17,000 are sheltering in the UN base in the town of Bor, now reportedly re-taken by the South Sudan army.
A further 7,000 people are sheltering at the UN base in Bentiu, state capital of the oil-producing Unity state — also in rebel hands — the UN added, while "sporadic fighting" has been reported in Upper Nile, another oil-rich state.
The UN said at least 45,000 people were sheltering at those and other bases.
"The estimated number of people displaced in the current crisis in South Sudan has risen to 81,000," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in its latest situation report.
"Given the limited access to civilians outside population centres, the number is likely to be significantly higher."
Ready for talks
South Sudan's sacked vice president Riek Machar said Tuesday he was ready for peace talks with his estranged mentor President Salva Kiir to bring an end to the deadly clashes across the country.
"Yes we are ready for talks. I have formed my delegation," he told Radio France Internationale (RFI), adding that the negotiations would likely be held in Ethiopia.
"I also spoke this morning to (US) Secretary of State John Kerry and I spoke to the foreign minister of the Federal Republic of Ethiopia, explained to him my readiness for talks," Machar said.
He said he would not take part in the talks but had formed a "very high-level delegation... with powers to reach agreement."
"We want a democratic nation. We want democratic free and fair elections. We want Salva Kiir to call it a day," he said.
Machar said the talks should be held on "neutral ground." Asked specifically if he was considering Ethiopia, he said: "That's the idea. Ethiopia, yes."